Set a 6-quart Dutch oven over medium-high heat. Add 2 tablespoons of the olive oil to the pan and once the oil is hot, crumble the sausage into the pan. Cook the sausage, stirring often until well browned on all sides, about 5 to 6 minutes. Add the tomato sauce to the pan and allow the sauce to simmer, for 45 minutes. Remove from the heat and set aside.
Bring a large pot of salted water to a boil. Add the spinach and cook for 2 minutes. Drain in a fine mesh strainer, pressing with a large spoon to release as much water as possible. Finely chop and set aside until ready to assemble the lasagna.
In a large skillet, melt the butter over medium-high heat. Add the shallots and garlic and cook, stirring, for 1 minute. Add the mushrooms and season with the salt and pepper. Cook, stirring, until the mushrooms are tender and have given off their liquid, about 5 to 7 minutes. Remove from the heat and cool. Set aside until ready to assemble the lasagna.
In a bowl, combine the 2 cups of the Bechamel sauce, the ricotta, Fontina, and mozzarella cheeses. Set aside until ready to assemble the lasagna.
Preheat the oven to 350 degrees F and grease a 15 1/2 by 11 1/2 by 4-inch roasting pan with the remaining 2 tablespoons of olive oil.
Ladle 4 cups of the Italian sausage-tomato sauce into the bottom of the pan. Lay 6 lasagna sheets across the top of the tomato sauce. Spread 1/2 of the spinach and half of the mushrooms over the top of the pasta, and pour 3 cups of the Parmesan Bechamel over the mushrooms and spinach. Sprinkle 1 cup of the Parmesan over the bechamel, as well as 1/2 the ricotta cheese mixture. Top with another layer of 6 lasagna sheets. Place 1/2 of the bolognese over the lasagna and top with another layer of pasta. Pour another 4 cups of the tomato sausage sauce over the pasta, and top with another layer of pasta. Spread the remaining spinach, mushrooms and bechamel over the pasta and sprinkle another cup of Parmesan over top. Top with the remaining ricotta cheese mixture and lay another row of pasta over the cheese mixture. Spread the remaining bolognese sauce over the pasta and top with another row of pasta sheets. Pour the remaining tomato sausage sauce over the pasta.
Wrap the top of the roasting pan with aluminum foil and set the roasting pan on a baking sheet. Bake covered for 1 1/2 hours, or until the pasta sheets are cooked through. Remove the roasting pan from the oven and remove and discard the aluminum foil. Increase the oven temperature to 400 degrees F. Sprinkle the remaining Parmesan over the top of the tomato sauce, and return the pan to the oven. Bake until the cheese is well browned, about 15 to 20 minutes.
Remove from the oven and allow lasagna to cool for at least 30 minutes before serving.
Yield: 12 to 20 servings